[7.5.3.14] \tau(\lambda)
\tau(\lambda) = \Phi_{\tau\lambda} / \Phi_{\mathrm{i}\lambda}
The ratio of the transmitted flux to the incident flux at a particular wavelength, λ, or within a small band of wavelengths, Δλ, about Δ.
Note: The various geometrical aspects of transmittance may each be considered restricted to a specific region of the spectrum and may be so designated by the addition of the adjective “spectral.”
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